Semiconductor-on-Insulator with Back Side Body Connection

ABSTRACT

Embodiments of the present invention provide for the removal of excess carriers from the body of active devices in semiconductor-on-insulator (SOI) structures. In one embodiment, a method of fabricating an integrated circuit is disclosed. In one step, an active device is formed in an active layer of a semiconductor-on-insulator wafer. In another step, substrate material is removed from a substrate layer disposed on a back side of the SOI wafer. In another step, an insulator material is removed from a back side of the SOI wafer to form an excavated insulator region. In another step, a conductive layer is deposited on the excavated insulator region. Depositing the conductive layer puts it in physical contact with a body of an active device in a first portion of the excavated insulator region. The conductive layer then couples the body to a contact in a second detached portion of the excavated insulator region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/459,110 filed on Apr. 28, 2012, which is a continuation ofU.S. patent application Ser. No. 12/836,506 filed Jul. 14, 2010, whichclaims the benefit of U.S. Provisional Patent No. 61/225,914 filed Jul.15, 2009. The content of U.S. Provisional Patent No. 61/225,914 isincorporated herein by reference.

FIELD OF THE INVENTION

The invention described relates to semiconductor-on-insulator devicesand processing generally, and more specifically to floating-body effectsin semiconductor-on-insulator devices.

BACKGROUND OF THE INVENTION

Semiconductor-on-insulator (SOI) technology was first commercialized inthe late 1990s. The defining characteristic of SOI technology is thatthe semiconductor region in which circuitry is formed is isolated frombulk substrate by an electrically insulating layer. This insulatinglayer is typically silicon-dioxide. The reason silicon-dioxide is chosenis that it can be formed on a wafer of silicon by oxidizing the waferand is therefore amenable to efficient manufacturing. The advantageousaspects of SOI technology stem directly from the ability of theinsulator layer to electronically isolate the active layer from bulksubstrate. As used herein and in the appended claims, the region inwhich signal-processing circuitry is formed on an SOI structure isreferred to as the active layer of the SOI structure.

SOI technology represents an improvement over traditional bulk substratetechnology because the introduction of the insulating layer isolates theactive devices in an SOI structure which improves their electricalcharacteristics. For example, the threshold voltage of a transistor isdesirously uniform, and is set in large part by the characteristics ofthe semiconductor material underneath the transistor's gate. If thisregion of material is isolated, there is less of a chance that furtherprocessing will affect this region and alter the threshold voltage ofthe device. Additional electrical characteristic improvements stemmingfrom the use of the SOI structure include fewer short channel effects,decreased capacitance for higher speed, and lower insertion loss if thedevice is acting as a switch. In addition, the insulating layer can actto shield the active devices from harmful radiation. This isparticularly important for integrated circuits that are used in spacegiven the prevalence of harmful ionizing radiation outside the earth'satmosphere.

SOI wafer 100 is shown in FIG. 1. The wafer includes substrate layer101, insulator layer 102, and active layer 103. The substrate istypically a semiconductor material such as silicon. Insulator layer 102is a dielectric which is often silicon-dioxide formed through theoxidation of substrate layer 101. Active layer 103 includes acombination of dopants, dielectrics, polysilicon, metal layers,passivation, and other layers that are present after circuitry 104 hasbeen formed therein. Circuitry 104 may include metal wiring; passivedevices such as resistors, capacitors, and inductors; and active devicessuch as transistors. As used herein and in the appended claims, the“top” of SOI wafer 100 references top surface 105 while the “bottom” ofSOI wafer 100 references bottom surface 106. This orientation schemepersists regardless of the relative orientation of SOI wafer 100 toother frames of reference, and the removal of layers from, or theaddition of layers to SOI wafer 100. Therefore, active layer 103 isalways “above” insulator layer 102. In addition, a vector originating inthe center of active layer 103 and extending towards bottom surface 106will always point in the direction of the “back side” of the SOIstructure regardless of the relative orientation of SOI wafer 100 toother frames of references, and the removal of layers from, or theaddition of layers to SOI wafer 100.

Semiconductor devices can be subject to a phenomenon known as thefloating-body effect. Semiconductor-on-insulator devices areparticularly susceptible to this effect. The manner in which thefloating-body effect is exhibited by an n-type field effect transistor(NFET) will be described for illustrative purposes, but thefloating-body effect is exhibited by many other active devices. FIG. 1Bdisplays a side-view of NFET 108. NFET 108 is an SOI device, and istherefore disposed above insulator layer 102. The floating-body effectis caused by the presence of excess carriers in body 109. Carriers canbuild up in body 109 through random generation of electron and holepairs by thermal or optical means, through scattering of high speedelectrons in channel 110, through leakage from source 111 or drain 112,through band-to-band tunneling, or through avalanche breakdown inchannel 110. The presence of excess carriers is therefore inevitable inany semiconductor device. However, in an SOI device, body 109 isisolated and limited as compared to a device whose body is part of bulksubstrate. Therefore, far fewer excess carriers are needed to alter thecharacteristics of the active device.

Two alterations to the characteristics of an active device caused by thefloating-body effect that are exacerbated by an SOI structure are thekink effect, and the non-linear capacitance exhibited by an activedevice that is in an off state. The introduction of excess carriers tobody 109 due to avalanche breakdown caused by a high potential appliedacross source 111 and drain 112 will have the effect of greatlyincreasing the current through channel 110. The effect is called thekink effect because the relatively flat portion on a curve of thechannel current against the drain-source potential will have a kinkupwards at the point where this effect takes hold. The relatively flatportion of the curve is located in a region where the current is—forsome applications—desirously set predominately by the voltage at gate113. This effect can therefore be problematic because certain analogcircuit applications are dependent upon the current of an active devicebeing independent of the drain-source potential when operating in thisregion.

In contrast to the kink effect, the non-linearity of a device'soff-state capacitances is not caused by avalanche breakdown. Instead,carriers build up through other less aggressive means as describedabove. If the potential of body 109 shifts to a significant enoughdegree, the capacitance seen by a signal at drain 112 will change in anon-linear fashion. The change will be non-linear because the excesscarriers will build up in body 109 over time making the capacitancetime-variant. Also, the charge build up will make the capacitance of thejunction between body 109 and drain 112 dependent upon the signal atdrain 112 which is also a characteristic of a non-linear system. Thiseffect can be problematic because certain circuit designs are dependentupon the retention of a highly linear characteristic for their processedsignals. For example, if NFET 108 was being used as a switch in aradio-frequency (RF) application wherein it had to be in an off statewhile a signal was transmitted on a line connected to drain 112, thecapacitance from drain 112 to body 109 would have to be linear in orderto prevent the production of unwanted harmonic distortion andinter-modulation distortion in the signal.

A common solution to the floating-body effect in SOI devices includesthe introduction of a connection from body 109 to source 111. Thissolution is a subset of the more general family of solutions involvingthe use of what is called a “body tie”, or “body contact”. A bodycontact provides a connection to body 109 which serves to remove excesscarriers. The particular solution of connecting body 109 to source 111is employed most commonly because it is so simple. Unwanted charge thatbuilds up in body 109 will be able to escape from body 109 to source111, and will therefore not cause the kink effect or lead to theproduction of a non-linear capacitance.

Another solution to the floating-body effect in SOI devices involves theuse of a smart body tie. A smart body tie is a body tie that changes itsstate based on the state of the device for which it is providing a tie.An example of a smart body tie can be described with reference to FIG.1C. FIG. 1C comprises an NFET 114. The source of NFET 114 is connectedto ground 115. The drain of NFET 114 is connected to drain contact 116.The gate of NFET 114 is connected to gate contact 117, and the cathodeof diode 118. The body of NFET 114 is connected to the anode of diode118. A similar configuration could function by replacing NFET 114 with aPFET and reversing the polarity of diode 118. This structure isadvantageous in certain situations because the body tie formed by diode118 will conduct much more when the device is off as compared to whenthe device is on. This can be very helpful for the situation describedabove wherein a non-linear off-state capacitance of the FET would imbuea processed signal on drain contact 116 with distortion. When gatecontact 117 is low and the device is off, current will flow from thebody of NFET 114 to gate contact 117 through diode 118. However, whengate contact 117 is high, the path from the body to gate willeffectively be cut off. This can be highly advantageous given that thekink effect provides a benefit from the perspective of providing highercurrent during the device's on-state current. Therefore, this structureallows for the drawbacks of the floating body effect in one applicationto be eliminated while preserving the advantages of the floating bodyeffect.

Although these approaches have advantageous aspects in that they areable to remove excess charge from the body of an active device, they areat the same time slightly problematic because they generally requireanother layer of processing in close contact to the active devices. Thisadditional processing can complicate the fabrication process and cangenerally lead to non-idealities in the fabricated active devicesthrough manufacturing errors. In addition, these approaches requireadditional area on the active wafer which increases the cost of theoverall design. These approaches also suffer from the disadvantage ofhigh resistance along the width of the transistor from the body tie tothe most remote portion of the channel. High resistance can reduce theefficacy of the body tie in reducing floating body effects. Finally,these approaches may introduce parasitic capacitance to nodes of thedevice that will limit the speed of any circuit utilizing such a device.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a semiconductor-on-insulatorstructure is disclosed. The SOI structure comprises an electricallyconductive layer. The SOI structure additionally comprises an insulatorlayer located above the electrically conductive layer, and partiallyvertically coextensive with the electrically conductive layer in anexcavated insulator region. The SOI structure additionally comprises anactive layer located above the insulator layer, and comprising an activedevice with a body. The SOI structure additionally comprises a bodycontact physically connecting the electrically conductive layer and thebody, and is located in a first portion of the excavated insulatorregion. The electrically conductive layer couples the body contact to acontact in the active layer. In addition, the contact in the activelayer is located in a second portion of the excavated insulator region.In addition, the second portion is detached from the first portion.

In another embodiment of the invention, a method of fabricating anintegrated circuit is disclosed. In one step, an active device is formedin an active layer of a semiconductor-on-insulator wafer. In anotherstep, substrate material is removed from a substrate layer disposed on aback side of the SOI wafer. In another step, an insulator material isremoved from a back side of the SOI wafer to form an excavated insulatorregion. In another step, a conductive layer is deposited on theexcavated insulator region. Depositing the conductive layer puts it inphysical contact with a body of an active device in a first portion ofthe excavated insulator region. The conductive layer then couples thebody to a contact in a second detached portion of the excavatedinsulator region.

In another embodiment of the invention a method of removing unwantedaccumulated majority-type carriers from the channel of asemiconductor-on-insulator active device is disclosed. In a first step,unwanted accumulated majority-type carriers are conducted from saidchannel to a body contact of the active device. The body contactphysically connects an electrically conductive layer and a body of theactive device. In a second step, unwanted accumulated majority-typecarriers are conducted through a circuit branch in the electricallyconductive layer. The electrically conductive layer is located below aninsulator layer, and is partially vertically coextensive with theinsulator layer in an excavated insulator region. The circuit branchcouples the body contact to a contact. The body contact is located in afirst portion of the excavated insulator region, and the contact islocated in a second detached portion of the excavated insulator region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and 1B illustrate a block diagram of an SOI device that issusceptible to the floating-body effect that is in accordance with theprior art.

FIG. 1C illustrates a block diagram of an NFET with a smart body contactthat is in accordance with the prior art.

FIG. 2 illustrates a block diagram of an SOI structure with a back sidebody contact that is in accordance with the present invention.

FIG. 3 illustrates a block diagram of an SOI structure with a back sidebody contact coupled to a gate through a pn-junction diode that is inaccordance with the present invention.

FIG. 4 illustrates a block diagram of an SOI structure with a back sidebody contact coupled to a gate through a hot-carrier-junction diode thatis in accordance with the present invention.

FIG. 5 illustrates a block diagram of the back side of an SOI structurehaving large polysilicon contact pads that is in accordance with thepresent invention.

FIG. 6 illustrates a process flow chart of a method of fabricating anintegrated circuit having a back side body contact that is in accordancewith the present invention.

FIG. 7 illustrates a block diagram of an SOI structure that is inaccordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosedinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe present technology, not as a limitation of the present technology.In fact, it will be apparent to those skilled in the art thatmodifications and variations can be made in the present technologywithout departing from the spirit and scope thereof. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers such modifications andvariations as are within the scope of the appended claims and theirequivalents.

Embodiments of the present invention provide for the production of SOIdevices that have space-saving efficient back side body contacts.Embodiments of the invention achieve this result through the utilizationof back side processing, the removal of portions of the SOI buriedinsulator layer, and the deposition of an electrically conductivematerial which connects a body contact of an active device to a secondcontact located in the same active layer as the active device itself

FIG. 2 displays SOI structure 200 which is in accordance with thepresent invention. As with prior art SOI devices, active layer 103 isabove insulator layer 102. Electrically conductive layer 201 is belowinsulator layer 102, and is disposed on the back side of insulator layer102 such that it fills excavated insulator region 202. As seen in thefigure, insulator layer 102 is at least partially vertically coextensivewith electrically conductive layer 201 in excavated insulator region202. As used herein and in the appended claims, the term “region” is notmeant to be limited to the description of a single contiguous region.Excavated insulator region 202 therefore allows for separate physicalcontacts between the body 109 of active device 203 and electricallyconductive layer 201 at body contact 204, and between said electricallyconductive layer 201 and said active layer 103 at second contact 205.Active layer 103 is bonded to handle wafer 206 to provide support toactive layer 103 while it is being processed. However, handle wafer 206can be removed at a later time during processing. In addition, stabilityduring processing may be provided by other means such that handle wafer206 does not need to be attached at all.

Embodiments of the present invention which are described with referenceto FIG. 2 in the previous paragraph function to remove excess carriersfrom body 109 to alleviate the floating body effect for active device203. Excess carriers that build up in body 109 are able to flow outthrough electrically conductive layer 201 and back into active layer103. Handle wafer 206 allows for the processing of the SOI structurefrom the back side which enables easy access to the body of activedevice 203. Advantageously, active device 203 can be formed in activelayer 103 unimpeded by the need for body tie circuitry. As such, thebody tie circuitry is much less likely to adversely affect the finishedactive devices or hinder the flexibility of a designer that develops thelayout for the active devices. In addition, any parasitic capacitancethat results from having body tie circuitry close by can be greatlyalleviated because only a small contact is needed near the body of thedevice.

A specific embodiment of the present invention can be described withreference again to FIG. 2. In FIG. 2, a circuit branch comprising bothbody contact 204 and second contact 205 connects to both source 111 ofactive device 203, and body 109 of active device 203. In specificembodiments of the invention, the configuration shown in FIG. 2 allowsexcess carriers to flow to source 111 and then away from active device203 along the source electrode. This is a convenient configuration giventhat the source of NFETs will generally be at a lower potential thanbody 109 so p-type carriers will flow out through this circuit branch.In addition, the source of a p-type field effect transistor (PFET) willgenerally be at a higher potential than body 109 so n-type carriers willflow out through this circuit branch. Another specific embodiment of thepresent invention comprises a circuit branch comprising both bodycontact 204 and second contact 205. However, in contrast to theconfiguration shown in FIG. 2, the circuit branch that includes bodycontact 204 and second contact 205 connects gate 113 to body 109. Theresultant device is commonly referred to as a dynamic-threshold FET(DTFET or DTMOS). This configuration will function by providing a pathfor carriers to leave body 205 as they will be attracted to the signalsource for gate 210. The DTMOS device provides higher threshold voltagewhen the transistor is biased in the OFF condition and lower thresholdvoltage when the device is in the ON condition. This advantageouslyprovides low leakage when OFF and high drive strength when it is turnedON.

The back side processing concepts discussed above with reference to FIG.2 can be applied to the formation of smart body contacts to formstructures that are in accordance with the present invention. Inspecific embodiments of the present invention, a variable impedancecircuit branch comprising body contact 204 and second contact 205 isconfigured to have a high impedance when active device 203 is in an onstate, and a low impedance when active device 203 is not in an on state.Although these embodiments are described below with reference to NFETdevices the same result can be achieved by using a PFET device in placeof the described NFET device while also configuring any attached diodesto have opposite polarity.

An SOI structure 300 that is in accordance with the present inventioncan be described with reference to FIG. 3. In FIG. 3, gate 113 iscoupled to electrically conductive layer 201 through a diode comprisinganode 302 and cathode 301. In specific embodiments of the invention,anode 302 will comprise a region of active layer 103 that has been dopedmore using the same doping profile as body 109. In specific embodimentsof the invention, such as embodiments employing thin-film siliconprocesses, anode 302 and cathode 301 will be side-by-side in activelayer 103 rather than stacked vertically. The circuit branch comprisingthis diode has variable impedance based on the relative voltages of gate113 and body 109. In the situation where the potential of gate 113drops, the impedance of this variable impedance circuit branch will dropnearly to zero thereby latching body 109 to gate 113 when the gate islow.

Embodiments of the present invention which are described with referenceto FIG. 3 in the previous paragraph function to filter the benefits ofthe floating-body effect from the effect's drawbacks when active device203 is used in certain applications. These advantages are discussedabove with reference to prior art smart body contacts. As before withreference to FIG. 2, these benefits are achieved without having todisturb or alter the layout of active device 203. Therefore, the designof active device 203 can be altered to accommodate other concernsdecoupled from concerns regarding the floating-body effect. In addition,in specific embodiments of the invention the impedance of the variableimpedance circuit branch in its low impedance state can actually behigher than in some prior art body contacts and still retain efficacy.In embodiments of the present invention utilizing smart body contacts toprevent harmonic distortion of a signal passing above an off state RFswitch, the carriers that must be removed from the body are thosegenerated through much slower processes than those generated byavalanche breakdown and impact ionization. The relevant processes areseveral orders of magnitude slower and therefore the generated chargecan be removed even through a high impedance path. Therefore, thecircuitry that comprises the variable impedance path lying within theactive layer can be kept to a minimal size for a commensurately minimalimpact on the overall size and cost of a device using SOI structure 300.

A specific embodiment of the present invention can be described withreference again to FIG. 3. In FIG. 3, body contact 204 is ohmic andthere is negligible resistance from body contact 204 to second contact205. Also, second contact 205 is directly below the diode comprised ofcathode 301 and anode 302. Since active device 203 is an NFET the gatewill go high during its on state. Therefore, anode 302 will be at alower potential than cathode 301 and the diode will be reversed biased.This will prevent carriers from departing body 109 which will providethe NFET that is active device 203 to support a higher drive current andprevent forward bias of the body to source diode, which would cause highcurrents to flow in an undesired path. If active device 203 was a PFETthe gate would go low during its on state. Therefore, anode 302 andcathode 301 would need to be switched in FIG. 3. If this configurationwere applied, the diode would be reversed biased when the PFET was inits on state and the device 206 could support a higher drive current inits on state, and prevent forward bias of the body to source diode.Preventing forward bias of this diode is important to avoid the flow ofhigh currents in an undesired path.

In specific embodiments of the present invention, the same bias schemeis provided to body 109 as described with reference to FIG. 3 usingalternative structures. For example, an independent voltage bias sourcecan be applied to body 109 through electrically conductive layer 201.This voltage bias source would provide a variable voltage depending uponthe voltage on gate 113 to provide the same effect as described withreference to FIG. 3. As another example, a FET could be coupled in aspart of variable impedance path to serve a similar function to thatprovided by the diode in FIG. 3, that is, the FET will provide a lowimpedance path from the body to the gate when the transistor is not inan on state, and will provide a high impedance path from the body to thegate when the transistor is in an on state. In specific embodiments ofthe invention, various other circuits and configurations are applied forremoving the charge from body 109 in accordance with the conceptsillustrated by FIG. 3.

An SOI structure 400 that is in accordance with the present inventioncan be described with reference to FIG. 4. FIG. 4 is similar to FIG. 2with the exception that the circuit branch from second contact 205rising up into active layer 103 is a variable impedance path 401. Inspecific embodiments of the invention, body contact 204 is a hot-carrierdiode which provides for the variable aspect of variable impedance path401. In specific embodiments where active device 203 is an NFET device,the hot-carrier diode would be reversed-biased when the potential ongate 113 was high, and forward biased if the potential on gate 113 waslow. These embodiments will therefore exhibit the same characteristicsdescribed above with reference to smart body contacts. Advantageously,this configuration does not require any active devices to be built up inactive layer 201. This will therefore save space given that variableimpedance path 401 can be implemented using only a single metal linepassing through active layer 103.

In specific embodiments of the invention, the metal used to formelectrically conductive layer 201 is used for other purposes as well.For example, the metal may be used as assembly metal to provide contactsto the active layer. Such contacts could be used to provide powersignals into the active layer. The contacts could also be used to routesignal lines to and from the active layer. As another example, the metalcan be run along the channel on the backside of the SOI structure todecrease the capacitance between body and source and between body anddrain. The reason this configuration will reduce these parasiticcapacitances is the metallization will not cross over the source ordrain silicon and will instead remain over the channel or body area.Therefore the parallel plate capacitance between the body metal line andthe source and/or drain is minimized. This will produce advantageousresults given that the speed and performance of a device is inherentlylimited by the size of its parasitic capacitances.

A specific embodiment of the invention exhibiting large channel contactscan be described with reference to FIG. 5. FIG. 5 illustrates the backsurface of SOI structure 500. Lithographic resolution and alignmentcapabilities of a fabrication process may preclude the creation of acontact to be used as body contact 204 with a high rate of success. Acritical design failure would result if nearly any FET in a design wasfaced with a situation where its drain and source where shorted togetherby the electrically conductive material used for body contact 204. Inspecific embodiments of the invention, larger landing pads 501 ofchannel material can be made to increase the width of channel 502 at thechannel contact locations by an amount sufficient to allow contact tothe channel while ensuring that source or drain are not contacted. Asseen in FIG. 5, the metal deposited to form backside contact 503 to formbody contact 204 has a larger area to make contact with channel 502 atlanding pad 501 thereby reducing constraints upon the manufacturingprocess and potentially increasing fabrication yield. One method thatcan be used to create a larger landing pad 501 includes widening thepolysilicon gate in that shape. A second method that can be used tocreate a larger landing pad 501 includes altering the source and drainimplant layout in that shape while keeping the polysilicon shapeunchanged. Although these embodiments slightly work against theadvantages of the present invention in terms of decoupling the bodycontact architecture from the transistor architecture, the modificationwould generally only need to be slight and would be minimal compared toprior art approaches that deposited body contacts directly under oradjoining the device itself in the active layer.

Methods of producing an integrated circuit that are in accordance withthe present invention can be described with reference to FIG. 6. In step600 an SOI structure undergoes processing to form an active device in anactive layer of the SOI wafer. The circuitry formed during this step andin this layer can include but is not limited to technologies such asCMOS, BiCMOS, SiGe, GaAs, InGaAs, and GaN. The circuitry can comprise:various active devices such as diodes and transistors; various passivedevices such as resistors, capacitors, and inductors; and routingcircuitry such as metal wires and vias. Various photolithographic andchemical deposition steps can be conducted to formulate this circuitry.Step 600 includes the formation of a gate of the active device. Step 600may also include the formation of contacts in the active layer that aredisposed on the bottom of the active layer vertically adjacent to theinsulator layer. These contacts may be made of channel material in thechannel region of the active device. In specific embodiments of theinvention, contacts may be made of areas of channel material that arewider than the remainder of a channel formed by the material. Theselarger areas could form landing pads for later connection to contactsdeposited from the back of the active wafer as described above. Step 600could also include the formation of contact separate from the activedevice, and a circuit branch that connects to a separate contact formedon the bottom of the active layer. This circuit branch could thenconnect to the source or gate of the active device. This circuit branchcould be used to form part of a variable impedance path or generalconductive path as described with regard to the devices described above.Finally, step 600 can also include the step of forming a diode in theactive layer. This diode could then be used to form part of the variableimpedance path described with regard to the devices described above.This diode can be placed directly above a contact but it can be placedanywhere in the active layer. The diode should be formed so as to matchthe necessary polarity for the devices described above to functionproperly in accordance with the present invention.

The remaining steps of the method illustrated by FIG. 6 involve backsideprocessing. These steps may be preceded by the attachment of a temporaryhandle wafer to the top side of the SOI wafer. This wafer can providestability to the SOI wafer while the remaining steps are carried out.However, as mentioned previously the attachment of this wafer is notnecessary as support can be provided through other means. The handlewafer can have an insulator layer that bonds through various means tothe top of the SOI wafer. However, the handle wafer may also have alayer that bonds to the top of the SOI wafer that is comprised ofsemiconductor material or a conductive material. In step 601, back sideprocessing commences with the removal of the SOI wafer substrate. Thesubstrate could be removed using mechanical and chemical meansindependently or in combination. For example, mechanical grinding can beused to thin the substrate material from an original thickness ofapproximately 800 micro-meters (μm) to approximately 20 μm. If thesubstrate is silicon, the final thickness of substrate material may beremoved with a wet etch such as KOH or TMAH. The final thickness ofsubstrate material may also be removed using a dry plasma etch. Thesubstrate can be removed with a high precision or etch rate ratio. Theetch rate ratio refers to the ratio of the portion of desired substratematerial that was removed from the back of the wafer to the portion ofadditional material that was removed which should not have been removed.In specific embodiments of the invention, the insulator layer is aburied-oxide that acts as an etch stop since the etch rate ratio isextremely high for the removal of all the substrate up to the buriedoxide.

In specific embodiments of the invention, the removal of substratematerial in step 601 is followed by the deposition of dielectrics orpassivation layers. In specific embodiments of the invention, step 601can be followed by the deposition of passivation layers to prevent ioniccontamination of the SOI structure. Finally, in specific embodiments ofthe invention, step 601 can be followed by the deposition of dielectriclayers which have the added benefit of reducing coupling capacitancebetween active devices in the active layer and the back sideelectrically conductive layer by spacing the electrically conductivelayer away from the active devices.

In step 602, insulator material is removed from the back of the SOIwafer to form an excavated insulator region. In specific embodiments ofthe present invention, this excavated insulator region is located belowa body of an active device in the active layer, and a second portion ofthis excavated insulator region is located below a separate portion ofthe active layer. In specific embodiments of the present invention, theexcavated insulator region may be located underneath the preparedcontacts that may have been produced in step 600. In specificembodiments of the invention, the separate portion of the active layermay be where a diode that was produced in step 600 is located. Step 602may involve the removal of additional layers including passivationdielectric. In general, the pattern of insulator removal will allow forthe deposition of an electrically conductive layer in step 603 that willallow electrical contact between any of the individual portions of theexcavated insulator region. These patterns can be formed using standardphotolithographic techniques and wet or dry etches.

In step 603, an electrically conductive layer is disposed on the backside of the SOI wafer. This layer can be applied in a patterned fashionto allow for connection between isolated sets of portions of theexcavated insulator region. This electrically conductive layer willcouple the body of an active device in one portion of the excavatedinsulator region to a separate portion of the excavated insulatorregion. The deposition of this electrically conductive material can bedone using electron beam sputtering, electroplating, electrolessplating, selective chemical vapor deposition, and various other methods.In specific embodiments of the invention, step 603 can be accompanied bythe deposition of various other layers to provide improved thermalcharacteristics to the SOI structure. In specific embodiments of theinvention, step 603 can be accompanied by the deposition of multiplelayers in sequence providing barrier metal layers, anti-reflectioncoatings, and various other layers.

In specific embodiments of the invention, the electrically conductivematerial deposited in step 603 is patterned across the back of the SOIstructure. The electrically conductive material can be patterned to runparallel or perpendicular with the channel of an active device in theactive wafer. However, the electrically conductive material can bepatterned independently of the channel orientations as well. In thespecific embodiments where the electrically conductive material ispatterned to run parallel with the channel, the overall capacitance seenby the active device will be decreased.

In specific embodiments of the invention, the electrically conductivematerial deposited in step 603 forms a hot-carrier diode junctionbetween the active layer and the electrically conductive material at thebody contact. In situations where the active device is a NFET and thebody is thereby p-type, various materials can be used to create thisdevice. A nonexclusive list of metals that can be used includes,aluminum, titanium, gold, palladium, nickel, platinum, and cobalt. Ifthe device is a PFET and the body is thereby n-type, the samenonexclusive list includes, aluminum, platinum, chromium, gold, andhafnium. The benefits of the resulting configurations are describedabove. In order to reduce sheet resistance, a thin layer of the metalused to create the hot-carrier diode can first be deposited, followed bya layer of lower resistivity metal such as aluminum or copper.

In specific embodiments of the invention, the electrically conductivematerial deposited in step 603 is additionally used for assembly. Theelectrically conductive layer could comprise solder bumps, copper posts,or other types of packaging materials. This assembly metal could be usedto provide power to the circuit in the active layer of the SOIstructure, and could additionally route signals to and from the activelayer of the SOI structure. This assembly material could also bedisposed in a separate deposition after the electrically conductivelayer used to generate the body contact has already been deposited.

An SOI structure 700 that is in accordance with the present inventioncan be described with reference to FIG. 7. SOI structure 700 compriseselectrically conductive layer 201. Electrically conductive layer 201comprises a single contiguous region of material. Electricallyconductive layer 201 is at least partially vertically coextensive withinsulator layer 102 in excavated insulator region 202. Electricallyconductive layer 201 provides a low barrier contact to both body 109 andsource 111. Therefore, body 109 is tied to source 111 with a singlecontact. A structure in accordance with these embodiments can beproduced using methods described with reference to FIG. 6 with specificpatterns applied for substrate removal in step 601 to expose the bottomof source 111.

Specific embodiments of the invention that are in accordance with FIG. 7exhibit advantageous characteristics. With reference to FIG. 5, theseembodiments exhibit advantageous characteristics because the back sidecontact will naturally already have a larger landing pad to work withgiven that the single contact for electrically conductive layer 201 willbe disposed on both body 109 and source 111. In addition, in specificembodiments the tie does not take up any room in active layer 103because the tie exists solely within excavated insulator region 202. Inspecific applications, embodiments in accordance with FIG. 7 will beeasier to manufacture than other embodiments discussed above.

Although embodiments of the invention have been discussed primarily withrespect to specific embodiments thereof, other variations are possible.Various configurations of the described system may be used in place of,or in addition to, the configurations presented herein. For example,although the devices were discussed often with reference to siliconsubstrates and oxide insulator layers the invention will function withany form of semiconductor-on-insulator wafers, structures, or devices.For example, the invention will function in combination withsilicon-on-sapphire structures. In addition, the invention can functionor operate upon circuitry using any form of technology such as CMOS,bipolar, BiCMOS, SiGe, Ga,As, InGaAs, GaN and any other form ofsemiconductor technology or compound semiconductor technology. Inaddition, there may be additional layers of materials disposed betweenthose layers mentioned herein. Semiconductor processing is a highlydetailed field, and layers were only mentioned herein if they wereabsolutely necessary to describe the invention to avoid confusion. Forexample, there may be layers of passivation disposed on the active layerto prevent the circuitry from reacting with its environment. Inaddition, the use of the word “layer” such as when describing an activelayer or a insulator layer does not preclude such layers being comprisedof more than one material. For example, there may be layers of glass orsome other insulator below metal lines in active circuitry in additionto a silicon-dioxide insulator beneath the entire active layer of an SOIstructure. However, the term insulator layer can cover the entirestructure of the glass and silicon-dioxide insulator.

Those skilled in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the invention.Nothing in the disclosure should indicate that the invention is limitedto systems that require a particular form of semiconductor processing orto integrated circuits. Functions may be performed by hardware orsoftware, as desired. In general, any diagrams presented are onlyintended to indicate one possible configuration, and many variations arepossible. Although the disclosure was focused on the application of theinvention to FET devices the invention will also help to alleviateparasitic issues for BJT devices in SOI architectures. Those skilled inthe art will also appreciate that methods and systems consistent withthe present invention are suitable for use in a wide range ofapplications encompassing any related to the accumulation of chargecarriers in a specific region of an electronic device.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those skilled in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims.

The invention claimed is:
 1. A method of fabricating an integratedcircuit, the method comprising the steps of: forming an active device inan active layer of a semiconductor-on-insulator wafer; removing asubstrate material from a substrate layer disposed on a back side of thesemiconductor-on-insulator wafer; removing an insulator material from aback side of the semiconductor-on-insulator wafer to form an excavatedinsulator region; and depositing an electrically conductive layer on theexcavated insulator region; wherein the depositing puts the electricallyconductive layer in physical contact with a body of an active device ina first portion of the excavated insulator region, and couples the bodyto a contact, the contact being in a second detached portion of theexcavated insulator region.
 2. A method of fabricating an integratedcircuit from claim 1, further comprising the step of forming a handlewafer above the active layer.
 3. The method of fabricating an integratedcircuit from claim 1, further comprising the step of selecting a lowerresistivity metal to form at least a portion of the electricallyconductive layer.
 4. The method of fabricating an integrated circuitfrom claim 1, further comprising the step of forming a circuit branchfrom the contact to a source of the active device.
 5. The method offabricating an integrated circuit from claim 1, wherein the depositingforms a segment of a variable impedance circuit branch configured tohave a high impedance when the active device is in an on state, and havea low impedance when the active device is not in the on state.
 6. Themethod of fabricating an integrated circuit from claim 1, furthercomprising the step of forming a gate region of the active device and asecond segment of a variable impedance circuit branch from the gate tothe contact.
 7. The method of fabricating an integrated circuit fromclaim 1, further comprising the step of forming a diode in the activelayer directly above the contact, the diode configured to be reversedbiased when the active device is in an on state.
 8. The method offabricating an integrated circuit from claim 1, wherein the depositingforms a hot-carrier diode junction between the active layer and theelectrically conductive layer.
 9. The method of fabricating anintegrated circuit from claim 1, wherein the step of forming the activedevice in the active layer further comprises forming a landing pad abovea point where the body is contacted by the electrically conductivelayer, the landing pad being comprised of a wider region of the body.10. The method of fabricating an integrated circuit from claim 1,further comprising the step of depositing an assembly material, theassembly material being electrically coupled to the electricallyconductive layer.
 11. The method of fabricating an integrated circuitfrom claim 10, wherein the assembly material comprises solder bumps orcopper posts.